Submicron alpha alumina high temperature bonded abrasives

ABSTRACT

A high temperature bonded abrasive includes alumina abrasive grits, and a vitreous bond matrix in which the alumina abrasive grits are distributed, the vitreous bond matrix having a cure temperature not less than 1000° C. The alumina abrasive grits include polycrystalline alpha alumina having a fine crystalline microstructure characterized by an alpha alumina average domain size not greater than 500 nm, and the alumina abrasive grits further include a pinning agent that is a dispersed phase in the polycrystalline alpha alumina

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication No. 60/870,740, filed Dec. 19, 2006, entitled “SUBMICRONALPHA ALUMINA HIGH TEMPERATURE BONDED ABRASIVES”, naming inventors RalphBauer and Margaret L. Skowron, which application is incorporated byreference herein in its entirety.

BACKGROUND

1. Field of the Disclosure

Aspects of the present invention are generally directed to hightemperature bonded abrasive tools and components, particularly, hightemperature bonded abrasives that incorporate fine microstructurealumina abrasive grits.

2. Description of the Related Art

High performance abrasive materials and components have long been usedin various industrial-machining applications, includinglapping/grinding, in which bulk material removal is executed, to finepolishing, in which fine micron and submicron surface irregularities areaddressed. Typical materials that undergo such machining operationsinclude various ceramics, glasses, glass-ceramics, metals and metalalloys. Abrasives may take on any one of various forms, such as freeabrasives as in an abrasive slurry in which loose abrasive particles insuspension are used for machining. Alternatively, abrasives may take theform of a fixed abrasive, such as a coated abrasive or a bondedabrasive. Coated abrasives are generally categorized as abrasivecomponents having an underlying substrate, on which abrasive grits orgrains are adhered thereto through a series of make coats and sizecoats. Bonded abrasives typically do not have an underlying substrateand are formed of an integral structure of abrasive grits that arebonded together via a matrix bonding material.

State of the art bonded abrasives take advantage of vitreous bondingmaterials, such as silica-based glass bonding matrices. Alternatively,specialized bonded abrasives for certain applications incorporatesuperabrasive grits, such as cubic boron carbide and diamond, and may beintegrally bonded through the use of a metal alloy bond matrix.

While bonded abrasives have continued to undergo development in recentyears, particular attention has been paid to high temperature bondedabrasives that utilize a bonding matrix formed of a vitreous material.An example of a high temperature bonded abrasive component is describedin U.S. Pat. No. 5,282,875. While state of the art high temperaturebonded abrasive components have improved performance and durability, aneed continues to exist in the art for further improved components.

SUMMARY

According to one aspect, a high temperature bonded abrasive is providedthat includes alumina abrasive grits and a vitreous bond matrix in whichthe abrasive grits are distributed. The vitreous bond matrix has hightemperature properties, including a cure temperature not less than about1000° C. The alumina abrasive grits comprise polycrystalline α-aluminahaving a fine crystalline microstructure characterized by an α-aluminaaverage domain size not greater than 500 nm. The alumina abrasive gritsfurther comprise a pinning agent, the pinning agent comprising adispersed phase in the polycrystalline α-alumina phase.

According to another aspect, a high temperature bonded abrasive isprovided that includes alumina abrasive grits and a vitreous bond matrixin which the grits are distributed. The vitreous bond matrix has a curetemperature not less than 1000° C. The alumina abrasive grits comprisepolycrystalline α-alumina having a fine crystalline microstructurecharacterized by an average domain size of not greater than 300 nm.Further, the alumina abrasive grits comprise a pinning agent includingat least a zirconium oxide phase dispersed in the polycrystallineα-alumina phase.

In addition, a method for forming a high temperature bonded abrasive isprovided. The method calls for forming fine crystalline microstructureα-alumina abrasive grits by heat-treating α-alumina precursor containinga pinning agent at a temperature not less than 1350° C. A shaped body isthen formed containing the α-alumina grits and a vitreous bond matrixmaterial. Further, heat treatment of the shaped body is carried out at acure temperature not less than 1000° C. and above the melting point ofthe vitreous bond matrix material. The alumina abrasive grits have anaverage crystalline domain size not greater than about 300 nm afterheat-treating.

DESCRIPTION OF THE EMBODIMENTS

According to an embodiment, a high-temperature bonded abrasive includesalumina abrasive grits that have a particularly fine microstructure anda vitreous bond matrix in which the alumina abrasive grits aredistributed.

Turning first to a description of the alumina abrasive grits, typicallythe alumina abrasive grits are principally formed of polycrystallineα-alumina. The polycrystalline α-alumina generally forms the majorityphase of the grits, that is, at least 50% by weight. However, generallythe alumina abrasive grits are at least 60 wt. %, oftentimes at least 80wt. %, and in certain embodiments at least 90 wt. % polycrystallineα-alumina. The polycrystalline α-alumina has a fine crystallinemicrostructure that may be characterized by an α-alumina average domainsize not greater than 500 nm. The crystalline domains are discrete,identifiable crystalline regions of the microstructure that are formedof an aggregation of single crystals, or may be formed of a singlecrystal. However, according to certain embodiments, the crystallinedomains are monocrystalline and are easily observed through scanningelectron microscopy analysis. The crystalline domain size may be evenfiner, such as not greater than 400 nm, or not greater than 300 nm. Withthe even finer crystal domain size, typically the domains are singlecrystalline as noted above. Such fine domains may be particularly small,such as not greater than 200 nm, not greater than 190 nm, or even notgreater than 180 nm. It is noteworthy that the fine crystalline domainsize is present in the high temperature bonded abrasive component,post-processing. This is particularly noteworthy, as oftentimes theprocess for forming the high temperature bonded abrasive involves hightemperature treatment at which the vitreous bond matrix cures. Such hightemperature treatment has a tendency to cause domain growth, which isparticularly undesirable. Further details are provided below.

As noted above, the alumina abrasive grits further include a pinningagent. A pinning agent is a material that is foreign to the α-aluminamicrostructure of the grits, and can be identified by a second phasedispersed in the polycrystalline α-alumina matrix phase. The pinningagent is generally effective to “pin” the domains, thereby preventingexaggerated domain growth during sintering and/or high temperatureprocessing of the grits to form the bonded abrasive component. Examplesof a pinning agent include oxides, carbides, nitrides and borides, aswell as reaction products thereof with the polycrystalline α-aluminamatrix. According to particular embodiments, the pinning agent comprisesan oxide of at least one of silicon, boron, titanium, zirconium, and arare-earth element, and reaction products thereof with thepolycrystalline α-alumina matrix. A particular pinning agent iszirconium oxide, generally in the form of ZrO₂ (zirconia). Zirconiumoxide is particularly suitable material, and generally is inert withinthe polycrystalline α-alumina matrix, so as to undergo very limitedreaction with the alumina thereby retaining a zirconium oxide crystalphase, typically zirconia. The pinning agent is generally present in thealumina abrasive grits in an amount not less than about 0.1 wt. %, suchas an amount not less than about 0.5 wt. %, or not less than about 1.0wt. %. The lower limit of the pinning agent is chosen to be an amountthat is effective to prevent exaggerated domain growth.

According to one embodiment, the pinning agent is present in theabrasive grits in an amount not greater than 40 wt. %, such as an amountnot greater than 30 wt. %, not greater than 20 wt. % or even not greaterthan 10 wt. %. In the high temperature bonded abrasive, the pinningagent is generally identified as having a particulate size not greaterthan 5 microns, such as not greater than 1 micron. Fine particulatesizes associated with the pinning agent have been found to be useful,such as not greater than 500 nm, or not greater than 300 nm, or notgreater than 200 nm. As described in more detail below, in the contextof methods for forming high temperature bonded abrasive components, thepinning agent may be introduced into the alumina abrasive grits in solidform, such as in sub-micron form, particularly including colloidal form.Alternatively, the pinning agent may be introduced into the aluminaabrasive grits or precursor thereof, such that upon high temperatureheat treatment the pinning agent precursor converts into a desiredcrystalline phase such as the desired oxide, carbide, nitride or boride.

Processing to form a high temperature bonded abrasive according toembodiments of the present invention generally begins with the formationof the alumina abrasive grits. According to a particular embodiment, thealumina abrasive grits are formed through a seeded process, in which anappropriate seeding material is combined with an α-alumina precursor,followed by heat treatment to convert the α-alumina precursor into thedesired α-alumina phase. The seeds may be formed in accordance with U.S.Pat. No. 4,623,364, in which seeded gel alumina dried precursor iscalcined to form α-alumina. The calcined α-alumina may be furtherprocessed such as by milling to provide an appropriate high-surface areaseed material. Typically, the surface area is quantified by specificsurface area (SSA), not less than 10 m²/g, typically not less than 20m²/g, such as not less than 30 m²/g, or not less than 40 m²/g.Particular embodiments have a surface area not less than 50 m²/g.Generally, the surface area is limited, such as not greater than 300m²/g, such as not greater than 250 m²/g.

The seed material is then combined with the α-alumina precursor, whichmay take on any one of several forms of aluminous materials that is anappropriate form for conversion to α-alumina. Such precursor materialsinclude, for example, hydrated aluminas, including alumina trihydrate(ATH) and boehmite. As used herein, boehmite denotes alumina hydratesincluding mineral boehmite, typically being Al₂O₃.H₂O and having a watercontent on the order of 15%, as well as pseudo-boehmite, having a watercontent greater than 15%, such as 20% to 38%. As such, the term boehmitewill be used to denote alumina hydrates having 15 to 38% water content,such as 15 to 30% water content by weight. It is noted that boehmite,including pseudo-boehmite, has a particular and identifiable crystalstructure and accordingly, a unique X-ray diffraction pattern, and assuch, is distinguished from other aluminous materials, including otherhydrated aluminas.

Typically, the α-alumina precursor, such as boehmite is combined withthe seeded material such that the seeds are present in an amount notless than 0.2 wt. % with respect to total solids content of seeds andα-alumina precursor. Typically, the seeds are present in a amount lessthan 30 wt. %, or, typically, in an amount not greater than 20 wt. %.

The seeds and the α-alumina precursor are generally combined in slurryform, which is then gelled, such as by the addition of an appropriateacid or base, such as nitric acid. Following gellation, the gel istypically dried, crushed, and dried material is passed throughclassification sieves. The classified solid fraction may then besubjected to a sintering process that has limited soak time. Typically,sintering is carried our for a time period not exceeding 30 minutes,such as not greater than 20 minutes, not greater than 15 minutes.According to particular embodiments, the sintering period isparticularly short, such as not greater than 10 minutes.

According to a particular development, a pinning agent or pinning agentprecursor is added to the suspension containing seeds and α-aluminaprecursor. Typically, the pinning agent or pinning agent precursor ispresent in an amount not greater than 40 wt. % based upon the combinedsolids content of the α-alumina precursor, seeds, and pinning agent orpinning agent precursor (calculated based upon solids content of thepinning agent in the final α-alumina grit). Generally, the pinning agentis present in an amount not less than 0.1 wt. %, such as not less thanabout 0.5 wt. %, or even not less than about 1 wt. %, based upon thetotal solids content as noted above.

Still further, according to a particular development, sintering iscarried out at a temperature above the temperature that is necessary toeffect conversion of the α-alumina precursor into α-alumina. In a sense,certain embodiments call for “over-sintering” the α-alumina precursormaterial. Particularly suitable temperatures are generally not less than1350° C., such as not less than 1375° C., not less than 1385° C., notless than 1395° C. or not less than 1400° C. In this respect, it isnoted that while fine microstructured seeded α-alumina materials havebeen formed in the art, typically such materials are processed at lowertemperatures, oftentimes below 1350° C., such as on the order of 1300°C. Further observations on the combined effect of utilization of apinning agent and over-sintering are provided herein below.

Following sintering, optionally classified abrasive grits are thencombined with a vitreous bond material, shaped into an appropriategeometric contour (e.g., grinding wheel), which contours and shapes arewell appreciated in the context of the bonded abrasives art. Processingto complete the bonded abrasive component typically involves heattreatment at a cure temperature. As used herein, cure temperaturedenotes a material parameter associated with the vitreous bond matrixmaterial, and is generally in excess of the melting temperature, andparticularly, the glass transition temperature T_(g) of the bondingmaterial. The cure temperature is the minimum temperature at which thebond matrix material not only softens and becomes flowable, but alsobecomes flowable to an extent ensuring complete wetting and bonding tothe abrasive grits. Typically, the cure temperature according toembodiments herein is not less than 1000° C., generally indicatingformation of a high temperature bonded abrasive.

Particular examples were made according to the following description.

EXAMPLES Example 1 Comparative

In a 400 ml pyrex beaker, 30 grams of aluminum oxide hydroxide (pseudoboehmite) powder under the trade name DISPERAL obtained from Sasol Incof Hamburg Germany was stirred into 61 milliliters of deionized water(resistivity 2 mega-ohm cm).

As a seed feedstock, seeded gel alumina dried precursor as prepared inU.S. Pat. No. 4,623,364 was calcined at 1100 C for 5 minutes in a rotarykiln to convert the alumina to a form with a surface area as measured bythe BET method of 15 to 28 m²/g. 72 kg of this α-alumina feedstock wasmixed with 150 kg of deionized water and fed into a horizontal bead millmanufactured by Netzsch Company (headquarters Selb, Germany). The modelof the device was LMZ-25. Milling was conducted for 24 hours with theslurry continuously circulating through the mill. Approximately 40 kg ofalumina abrasive as manufactured by Saint-Gobain in size 46 grit wasemployed as the milling media. After milling the surface area was about75 m²/g.

To the slurry of Disperal, 1.43 grams of the seed slurry prepared abovewas added with stirring.

To this mixture was then added with stirring, 7.5 grams of 20% by weightHNO3 solution while stirring was continued the resultant mixture formeda gel.

The gel was dried overnight at 95° C. and then crushed with a woodenrolling pin. The fraction of grits passing through a 30 mesh sieve andremaining on a 45 mesh sieve was retained.

5 grams of the retained grits were then placed in an alumina boat andplaced into a preheated tube furnace (Lindberg Blue M series STF 55433)for sintering. Sintering was conducted for a total of 5 minutes.

Specimens were sintered at 3 different temperatures, 1300° C., 1350° C.and 1400° C. Hardness and crystal size properties were measured.

Bonded abrasive components of the sintered grits containing a vitreousbond matrix were made by mixing 1.22 grams of the sintered grits with0.72 grams of powder glass and adding 2 drops of 7.5% by weight Polyvinyl alcohol (PVA) solution. The composition of the glass powder isgenerally silica based, having a majority component of silica. Typicalsilica content is not less than 50% by weight, typically not less than60% by weight, such as not less than 65% by weight. Additionalcomponents of the glass powder include oxides such as alumina, sodiumoxide, magnesia, potassium oxide, lithium oxide, boron oxide, titania,iron oxide, calcia, other oxides and combinations thereof. Theparticular composition of the glass powder forming the bond matrix ischosen to have a desirably high cure temperature and Tg as discussed indetail above. The mixture was then placed into a 1.25 cm stainless steelmold and pressed at 10,000 psi. The resultant disc was then placed intoa cool muffle furnace (Lindberg type 51524) and heated up to 1250° C. in8 hours, held at this temperature for 4 hours and then cooled in 8hours. The resultant disc was prepared as a polished section andhardness and crystal size measured.

Example 2

This example illustrates the effect of a ZrO₂ pinning agent to preventundesirable crystal growth and to provide corrosion resistance.

An abrasive grit ceramic body was prepared as in Example 1 except thatcolloidal ZrO₂ was added at levels of 0.5% by weight relative to thefinal alumina value, 1.0% relative to the final alumina value and 2.0%relative to the final alumina value. The source of ZrO₂ was NYACOL 20 nmcolloidal ZrO₂ acetate stabilized form obtained from Nyacol. Specimenswere prepared and measured as in Example 1.

Example 3

This example illustrates the effect of a SiO₂ pinning agent to preventundesirable crystal growth and to provide corrosion resistance.

An abrasive grit ceramic body was prepared as in Example 1 except thatcolloidal SiO₂ was added at levels of 0.5% by weight relative to thefinal alumina value, 1.0% relative to the final alumina value and 2.0%relative to the final alumina value. The source of SiO₂ was NYACOLcolloidal SiO₂ ammonia stabilized form obtained from Nyacol Inc,Ashland, Mass. Specimens were prepared and measured as in example 1.

Example 4

This example illustrates the effect of a Y₂O₃ pinning agent to preventundesirable crystal growth and to provide corrosion resistance.

An abrasive grit ceramic body was prepared as in Example 1 except asolution of yttrium nitrate was added at equivalent yttrium oxide levelsof 0.5% by weight relative to the final alumina value, 1.0% relative tothe final alumina value and 2.0% relative to the final alumina value.The source of yttrium nitrate was from Alrdich chemicals. Specimens wereprepared and measured as in Example 1

Example 5

This example illustrates the effect of the pinning/anti-corrosion agentin a composite body, which includes magnesium oxide. Material was madeas in example 2 with 2% by weight ZrO₂ and 1% by weight of MgO added asa solution of magnesium nitrate. This example also incorporates a cobaltoxide (0.08%) colored marker as a visual indicator of extent ofcorrosion. The cobalt oxide precursor used was cobalt nitrate.

Crystal domain size of the examples described above was then measured byscanning electron microscopy (SEM) of a polished section of theExamples. Magnification of 50,000× was typically used, and specimenswere thermally etched for 5 minutes at 100° C. below the sinteringtemperature and the crystal domain size is reported or obtained by theintercept method without statistical correction. According toembodiments herein, crystal domains are fairly stable at hightemperature, which may be quantified in terms of Crystal Stability.Crystal Stability is defined herein as the temperature at which thealumina abrasive grits undergo limited domain growth quantified byaverage domain size not greater than 500 nm, after 5 minutes of exposureat such temperature. Embodiments herein have a Crystal Stability of atleast 1400° C., such as at least about 1500° C.

In addition to quantification of crystal domain size, the extent ofcorrosion was quantified using several techniques. During formation ofthe high temperature bonded abrasive, the vitreous bond material matrixhas a tendency to penetrate and react with the alumina grits. Suchattack is highly undesirable, and may be measured in terms of hardness.Herein, hardness was measured by taking hardness data of a smallsintered body (about 0.5 mm) at its center and near the outer edge(approximately 15 to 30 microns from the outer edge). The well-knownVickers microindentation method is used with a 500 g load. It isobserved that as the ceramic body undergoes corrosion, the hardness nearthe outer (exposed) edge decreases as softer phases are formed byreaction with the molten silicate glass. Corrosion was also measured byincorporating a colored (marker) such as cobalt oxide, at a level ofseveral hundred ppm which forms a blue colored cobalt aluminate in thesintered body. The depth of corrosion can be monitored visually byobserving the fading of the cobalt aluminate blue color due to reactionwith the silicate phases. Additionally, and particularly noteworthy, thecorrosion may be quantified in terms of corrosion index, represented bythe average depth of Si penetration after exposure of the aluminaabrasive grits to molten silica glass at 1250° C. for 4 hours.Embodiments herein show a corrosion index not greater than 15 microns,such as not greater than 10 microns, or even not greater than 8 microns.

Characterization of the ceramic bodies synthesized in Examples 1-5 aresummarized in Tables 1 and 2.

TABLE 1 Illustrates resistance to crystal domain growth as a function oftemperature, reported in microns. Ex 5 (2% TEMP ZrO₂ + 1% C./Xtal Ex 2(ZrO2) Ex 3 (SiO2) Ex 4 (Y2O3) MgO + 0.08 size u Ex 0.5 1.0 2.0 0.5 1.02.0 0.5 1.0 2.0 CoO) 1300 0.15 ND ND ND ND ND ND ND ND ND ND 1350 ND0.12 0.13 0.13 0.10 0.09 0.07 0.17 0.15 0.15 0.14 1400 0.56 0.16 0.160.16 0.11 0.08 0.07 0.16 0.19 0.17 0.15 1500 >1 ND ND ND 0.15 0.20 0.130.43 0.42 0.28 ND

The results in Table 1 clearly show that Example 1 undergoes dramaticdomain growth as a function of temperature. The examples containingvarious amounts of pinning agents reduce the sensitivity to grain growthas a function of temperature and thus extend the useful temperaturerange of application.

TABLE 2 Resistance to Corrosion All Samples Sintered ≧ 97% ofTheoretical Density Ex 5 (2% ZrO₂ + 1% Ex 2 (ZrO2) Ex 3 (SiO2) Ex 4(Y2O3) MgO + 0.08 Ex 1400° C. 1500° C. 1500° C. CoO) 11400° C. 0.5 1.02.0 0.5 1.0 2.0 0.5 1.0 2.0 1400° C. Edge 20.3 20.7 20.6 21.3 20.1 17.613.6 19.7 20.0 20.6 20.4 Hardness Before Test (Gpa) Edge 16.3 16.0 18.219.4 14.5 12.5 11.8 19.0 18.8 17.5 19.0 Hardness After Test (Gpa) Si20-30 5 5 Penetration (EDS) (um)

* Example 1 doped with 0.08% by weight CoO as described in example 5

Table 2 illustrates that additives of zirconia or yttria clearly reducethe reactivity of the grain with glass as demonstrated by betterretention of hardness. Additionally, use of additives such as ZrO₂clearly minimize the penetration of invasion of SiO₂ into the grain andprovide a more stable grain in view of molten glass. Additionally, it isclearly observed that when a cobalt oxide additive is used as a colorindicator, the extent of reaction when additives such as ZrO2 isemployed is much lower as evidenced by the retention of the blue color.

According to embodiments herein, particularly desirablefine-microstructured high temperature bonded abrasive components havebeen provided. Such fine-microstructured components are notable over thestate of the art, which is generally limited to medium-microstructuredhigh temperature bonded abrasives as exemplified in U.S. Pat. No.5,282,875, which teaches, at best, a microstructure having 600 nmcrystal domains and α-alumina single crystals having a size of 350 nmand larger (see U.S. Pat. No. 4,744,802, incorporated by reference intoU.S. Pat. No. 5,282,875). While fine-microstructured alumina abrasivegrits have been utilized in the past for free abrasive applications,such fine microstructured alumina abrasive grits typically have not beenutilized in the context of high temperature bonded abrasiveapplications. Such fine-microstructured materials have been found todissolve in the vitreous bond matrix material during processing and/orundergo exaggerated crystal domain growth during heat treatmentassociated with such high temperature bonded abrasives. This is clearlyshown in Example 1 described above, in which an initiallyfine-microstructured material was shown to experience exaggerated domaingrowth and excessive corrosion.

While not wishing to be bound by any particular theory, it is believedthat the combination of the use of a pinning agent and over-sinteringthe abrasive grits results a highly stable grit that is resistant tomicrostructural change during high temperature processing such as hightemperature use applications. It is believed that the pinning agent iseffective to grain growth that is normally observed at elevatedtemperatures, while the over-sintering processes condition is believedto impart notably improved anti-corrosion characteristics and furtherenhance the pinning effect of the pinning agent. The higher sinteringtemperatures as described herein may result in grain boundary phasesthat are more resistant to corrosion by way of modified crystallizationof the grain boundary, or uniform distribution of the grain boundaryvolume and/or selective dissolution of certain grain boundary elementsinto the matrix. The over-sintering condition may also synergisticallyaffect the pinning agent in a manner to impart additional corrosionresistance into the abrasive grit. Whatever the mechanism, the observedeffects are clear in that the high temperature bonded abrasivecomponents according to embodiments herein exhibit stable corrosioncharacteristics.

It is further noted that prior art seeding technology has mentionedutilization of grain growth stabilizers, including various oxidestabilizers, as well as sintering temperatures above 1300° C. normallyassociated with sintering of seeded sol-gel alumina abrasive grits.However, such grain growth inhibitors and sintering temperatures havebeen taught in connection with the general manufacture of α-aluminamaterials, and it has been believed that such fine-microstructuredmaterials would also suffer from exaggerated grain growth and/orexcessive corrosion in the context of a high-temperature bonded abrasivecomponent. However, it was surprisingly discovered that the combinationof pinning agent materials and over-sintering conditions addressed themarked deficiencies of not only crystal domain growth during processingand use conditions of the high temperature bonded abrasive, but alsocorrosion resistance.

While embodiments of the invention have been illustrated and describedwith particularity, the invention is not intended to be limited to thedetails shown, since various modifications and substitutions can be madewithout departing in any way from the scope of the present invention.For example, additional or equivalent substituents can be provided andadditional or equivalent production steps can be employed. As such,further modifications and equivalents of the invention herein disclosedmay occur to persons skilled in the art using no more than routineexperimentation, and all such modifications and equivalents are believedto be within the scope of the invention as defined by the followingclaims.

1. A high temperature bonded abrasive, comprising: alumina abrasivegrits; and a vitreous bond matrix in which the alumina abrasive gritsare distributed, the vitreous bond matrix having a cure temperature notless than 1000° C., wherein the alumina abrasive grits comprisepolycrystalline alpha alumina having a fine crystalline microstructurecharacterized by an alpha alumina average domain size not greater than500 nm, and wherein the alumina abrasive grits further comprise apinning agent, the pinning agent comprising a dispersed phase in thepolycrystalline alpha alumina.
 2. The high temperature bonded abrasiveof claim 1, wherein the pinning agent comprises a ceramic material thatis an oxide, carbide, boride, or nitride, reaction products thereof withthe polycrystalline alpha alumina, and combinations thereof.
 3. The hightemperature bonded abrasive of claim 2, wherein the pinning agent is anoxide of at least one element from the group consisting of silicon,boron, titanium, zirconium, rare earth, reaction products thereof withthe polycrystalline alpha alumina, and combinations thereof.
 4. The hightemperature bonded abrasive of claim 3, wherein the pinning agentcomprises zirconium oxide.
 5. The high temperature bonded abrasive ofclaim 1, wherein the pinning agent is present in the alumina abrasivegrits in an amount not less than about 0.1 wt %. 6-8. (canceled)
 9. Thehigh temperature bonded abrasive of claim 1, wherein the pinning agenthas an average particulate size not greater than about 1 micron.
 10. Thehigh temperature bonded abrasive of claim 9, wherein the pinning agenthas an average particulate size not greater than about 500 nm.
 11. Thehigh temperature bonded abrasive of claim 1, wherein the aluminaabrasive grits have an average crystal domain size not greater than 400nm.
 12. The high temperature bonded abrasive of claim 11, wherein thealumina abrasive grits have an average crystal domain size not greaterthan 300 nm.
 13. The high temperature bonded abrasive of claim 12,wherein domains of the polycrystalline alpha alumina are singlecrystalline.
 14. The high temperature bonded abrasive of claim 13,wherein the alumina abrasive grains have an average crystal domain sizenot greater than 200 nm.
 15. The high temperature bonded abrasive ofclaim 14, wherein the alumina abrasive grains have an average crystaldomain size not greater than 190 nm.
 16. The high temperature bondedabrasive of claim 15, wherein the alumina abrasive grains have anaverage crystal domain size not greater than 180 nm.
 17. The hightemperature bonded abrasive of claim 12, wherein the alumina abrasivegrits have a Crystal Stability of at least 1400° C., where CrystalStability is the temperature at which the alumina abrasive grits undergodomain growth quantified by an average domain size not greater than 500nm, after 5 minutes of exposure at said temperature.
 18. The hightemperature bonded abrasive of claim 17, wherein the alumina abrasivegrains have a Crystal Stability of at least 1500° C.
 19. The hightemperature bonded abrasive of claim 1, wherein the alumina abrasivegrits have a Corrosion Index not greater than 15 μm, wherein CorrosionIndex is the average depth of Si penetration after exposure of thealumina abrasive grits to molten silica glass at 1250° C. for 4 hours.20. The high temperature bonded abrasive of claim 19, wherein theCorrosion Index is not greater than 10 μm.
 21. (canceled)
 22. The hightemperature bonded abrasive of claim 1, wherein the vitreous bond matrixhas a cure temperature not less than 1100° C.
 23. The high temperaturebonded abrasive of claim 1, wherein the vitreous bond matrix has a glasstransition temperature T_(g) not less than about 1000° C.
 24. The hightemperature bonded abrasive of claim 23, wherein the vitreous bondmatrix has a glass transition temperature T_(g) not less than about1100° C.
 25. (canceled)
 26. A high temperature bonded abrasive,comprising: alumina abrasive grits; and a vitreous bond matrix in whichthe alumina abrasive grits are distributed, the vitreous bond matrixhaving a cure temperature not less than 1000° C., wherein the aluminaabrasive grits comprising polycrystalline alpha alumina having a finecrystalline microstructure characterized by an average domain size notgreater than 300 nm, and wherein the alumina abrasive grits furthercomprise a pinning agent, the pinning agent comprising a zirconium oxidephase dispersed in the polycrystalline alpha alumina.
 27. A method offorming a high temperature bonded abrasive, comprising: forming finecrystalline microstructure alpha alumina abrasive grits, by heattreating a alpha-alumina precursor containing a pinning agent at atemperature not less than 1350° C.; forming a shaped body containing thealpha alumina grits with a vitreous bond matrix material; and heattreating the shaped body at a cure temperature, the cure temperaturebeing not less than 1000° C. and above a melting point of the vitreousbond matrix material, the alumina abrasive grits having an averagecrystalline domain size not greater than 300 nm after heat treating. 28.The high temperature bonded abrasive of claim 27, wherein the alphaalumina precursor is heat treated in the presence of seeds.
 29. The hightemperature bonded abrasive of claim 28, wherein the seeds comprisealpha alumina seeds having an specific surface area not less than 10m²/g.
 30. The high temperature bonded abrasive of claim 28, wherein theseeds comprise alpha alumina seeds present within a range of about 0.2to about 20 wt % based on the alpha-alumina precursor solids content.